Rechargeables do not stay charged.
I work with NiCads in my tools and it takes good charging management to prevent being marooned. I've read that NiCads typically lose from 4 to 8% of their charge per day with a deceleration of loss as it nears complete drain.
I don't know the particularities of the battery the Sony uses but I'm quite sure that this is the phenomenon that we're noticing in this case.
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